XML 53 R42.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Unrealized gains (losses) on investments | $ 76.4 | $ 26.1 | $ 48.4 | $ 82.1
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments | (12.0) | (3.3) | (17.1) | (5.9)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax | 64.4 | 22.8 | 31.3 | 76.2
Foreign Currency Transaction Gain (Loss), Unrealized | (0.1) | (0.4) | (11.6) | (0.4)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax | 0.0 | 0.0 | 0.0 | 0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax | (0.1) | (0.4) | (11.6) | (0.4)
Net unrealized investment gains, pre-tax | 76.3 | 25.7 | 36.8 | 81.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax | (12.0) | (3.3) | (17.1) | (5.9)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax | 64.3 | 22.4 | 19.7 | 75.8
Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Unrealized gains (losses) on investments | 1.5 | 2.0 | (15.7) | 19.4
Foreign Currency Transaction Gain (Loss), Unrealized | 0.0 | 5.1 | (14.8) | 12.6
Net unrealized investment gains, pre-tax | 1.5 | 7.1 | (30.5) | 32.0
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Unrealized gains (losses) on investments | (0.1) | 0.0 | (0.1) | 0.0
Foreign Currency Transaction Gain (Loss), Unrealized | 0.0 | 0.0 | 0.0 | 0.0
Net unrealized investment gains, pre-tax | (0.1) | 0.0 | (0.1) | 0.0
Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Unrealized gains (losses) on investments | 54.9 | 26.6 | 43.7 | 53.5
Foreign Currency Transaction Gain (Loss), Unrealized | 0.0 | (3.1) | 0.0 | 0.0
Net unrealized investment gains, pre-tax | 54.9 | 23.5 | 43.7 | 53.5
Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Unrealized gains (losses) on investments | 20.1 | (2.5) | 20.5 | 9.2
Foreign Currency Transaction Gain (Loss), Unrealized | (0.1) | (2.4) | 3.2 | (13.0)
Net unrealized investment gains, pre-tax | $ 20.0 | $ (4.9) | $ 23.7 | $ (3.8)

X
- Definition Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30690-110894 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109240200&loc=d3e30700-110894 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 830 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6450222&loc=d3e30840-110895 + Details
Name: | us-gaap_ForeignCurrencyTransactionGainLossUnrealized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ScheduleOfTradingSecuritiesAndOtherTradingAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References No definition available. + Details
Name: | us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The after-tax aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
Name: | wtm_NetUnrealizedGainLossAndForeignCurrencyGainLossOnTradingSecuritiesNetOfTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
Name: | wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax + References No definition available. + Details
Name: | wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecuritiesTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the income taxes attributable to unrealized gains (losses) on trading securities. + References No definition available. + Details
Name: | wtm_TradingSecuritiesTaxonUnrealizedHoldingGainLossonInvestments
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The after-tax gain (loss) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period. + References No definition available. + Details
Name: | wtm_TradingSecuritiesUnrealizedForeignCurrencyGainLossNetOfTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The tax expense (benefit) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period. + References No definition available. + Details
Name: | wtm_TradingSecuritiesUnrealizedForeignCurrencyTransactionGainLossTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Represents the net unrealized investment gains (losses) after income taxes. + References No definition available. + Details
Name: | wtm_TradingSecuritiesUnrealizedHoldingGainLossonInvestmentsafterTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |